1. Field of the Invention
The invention generally relates to pulse-width modulated (PWM) power supplies; and in particular, the present invention relates to PWM power supplies with reduced electromagnetic interference (EMI).
2. Background of the Invention
Pulse-width modulated (PWM) power supplies encompass a wide variety of switching regulators having a constant switching frequency with variable "on" times (pulse widths). Adjusting the pulse widths based on a feedback voltage is used to generate a regulated dc voltage.
Exemplary circuit schematics for switching regulators can be found in Horowitz and Hill, "The Art of Electronics", 2.sup.nd ed., Cambridge University Press, 1989, pages 355-367, which is incorporated herein by reference in its entirety. Some other switching regulator topologies can be found in U.S. Pat. No. 4,842,532 to Freedman, U.S. Pat. No. 4,823,070 to Nelson, U.S. Pat. No. 5,170,333 to Niwayama, and U.S. Pat. No. 5,028,861 to Pace et al., which are incorporated herein by reference in their entireties.
In brief, conventional PWM regulators include a PWM controller, one or more switching transistors, and a filter for providing the regulated voltage output. An oscillator in the PWM controller controls the switching frequency of the switching transistors.
It is well known that the switching action of a conventional switching regulator generates a substantial amount of "noise," including conducted and radiated electromagnetic interference (EMI). Conducted EMI may be caused by output voltage or current ripple at the switching frequency. Radiated EMI is noise generated by the switching of the transistor(s) radiated into the environment at the switching frequency and its associated harmonics. Radiated EMI causes interference with radio transmissions in the vicinity of the regulator.
Because EMI noise generated by a switching regulator, either conducted or radiated, can interfere with the operation of nearby electronic equipment, design consideration of a switching regulator or its application must include careful management of spurious EMI emissions. Additionally, the Federal Communications Commission (FCC) imposes limits on the amount of conducted and radiated emission generated by electronic devices acting as unintentional radiators. (See, FCC regulations on Radio Frequency Devices, 47 C.F.R. .sctn.15.109.)
The substantial amount of spurious EMI emissions generated by conventional switching regulators sometimes limit their applications in electronic devices. Therefore, it is desirable to provide a method for reducing the EMI noise generated by switching regulators. It is further desirable to provide a switching regulator where unwanted EMI emissions are reduced to a level which meets the FCC regulations so that the switching regulator can be applied to a wide-range of electronic applications such as cellular telephones and hand-held instruments.